In the US there is common-law trademark protection http://www.bitlaw.com/trademark/common.html http://www.bitlaw.com/trademark/common.html It is not as strong

In the US there is common-law trademark protection http://www.bitlaw.com/trademark/common.html http://www.bitlaw.com/trademark/common.html It is not as strong as have a federally-registered trademark. But, as others have pointed out, the problem is in showing the likelihood of confusion. It's _possible_ that this musician could prove this, but I believe it can be quite expensive, and there's no assurance of winning. My gut feeling is the the idea of confusion is not so clear cut. If you apply for a trademark you need to specify the specific field of application, and there are quite a few and remarkably segmented. Even having a registered trademark as a musician may not stop another from getting a trademark on a tech conference. His best bet might have been to try to ride the wave of inadvertent attention, promoting his stuff to people on Twitter who followed him by mistake. Perhaps some number of them would have become fans/customers. But I can understand the anger when you think someone is stepping on your identity.